Phase difference plates used in liquid crystal displays have been widely used for attaining high contrast ratios and improving color shift phenomena at wide view angles in color TFT liquid crystal displays of various kinds of display modes, and the like. The types of the phase difference plates include, for example, a ¼ wavelength plate (hereinafter, abbreviated to as “λ/4 plate”) that converts linearly polarized light into circularly polarized light, and a ½ wavelength plate (hereinafter, abbreviated as “λ/2 plate”) that rotates the polarization vibration surface of linearly polarized light by 90°. Conventional phase difference plates are capable of adjusting monochromatic light to a phase difference of λ/4 or λ/2 with respect to light wavelength. However, the conventional phase difference plates have a problem in that white light, which is a synthesized wave and coexists with light beam in visible light region, is converted into colored polarized light due to distributions for polarization states at the respective wavelengths. This is caused by the fact that a material constituting a phase difference plate has wavelength dispersion (chromatic dispersion property) for phase difference.
For solving such a problem, various kinds of broadband phase difference plates capable of providing a uniform phase difference with respect to a wide-wavelength light have been proposed. For instance, there is disclosed a phase difference plate obtained by bonding a ¼ wavelength plate where the phase difference of birefringent light is ¼ wavelength with a ½ wavelength plate where the phase difference of birefringent light is ½ wavelength, while intersecting their optical axes (see, for example, JP-A-10-68816 (“JP-A” means unexamined published Japanese patent application)). In addition, there is disclosed a phase difference plate constructed of at least two phase difference plates having optical phase difference values of 160 to 320 nm, which are laminated at an angle that allows slow phase axes thereof to be neither parallel nor perpendicular to each other (see, for example, JP-A-10-90521).
However, for manufacturing the above phase difference plates, a complicated process is required for regulating the optical directions (optical axes and slow phase axes) of the two polymer films. The optical direction of the polymer film typically corresponds to the perpendicular or horizontal direction of a sheet or roll film. The polymer film having an optical axis or a slow phase axis in the diagonal direction of a sheet or roll is difficult to be industrially produced in large quantities. Further, it is necessary to set the optical directions of the two polymer films at an angle which does allow them to be neither parallel nor perpendicular to each other. Therefore, phase difference plates should be manufactured by carrying out the steps of obtaining chips by cutting two different polymer films at predetermined angles and then bonding the chips together, to give the phase difference plates. The steps of bonding chips and so on are complicated in operation and cause a decrease in quality due to axial displacement and a decrease in yield, while increasing costs and facilitating deterioration by contamination. In addition, it is also difficult to strictly regulate the optical phase difference values of polymer films, and the qualities thereof may tend to be decreased and so on.
For solving such a problem, there is proposed a method of manufacturing a broadband λ/4 plate with a single phase difference plate, without a lamination of phase difference plates (see, for example, WO00/26705).
The method can be preceded by mono-axial orienting using a polymer film obtained by copolymerizing a monomer unit of a polymer having positive refractive index anisotropy with a monomer unit of a polymer having a negative birefringence. Since the oriented polymer film has reverse wavelength dispersion characteristics, it is possible to prepare a broadband λ/4 plate using one phase difference film, thereby solving the above problems. However, the obtained phase difference values are within a narrow range, so many films should be laminated otherwise the sufficient optical characteristics cannot be obtained. As a result, a polarizing plate to be prepared is made thick and heavy. In addition, in the step of laminating films, there are needs of preventing the optical axes from being displaced, preventing exogenous materials from contamination, and the like. Further, there is also a problem that the producing method is complicated.
Further, in JP-A-2005-242293, a reverse dispersion wavelength film is formed by applying a polymerizable liquid crystal compound onto the upper layer of a cellulose acylate film, drying and polymerizing the compound, peeling a hardened film thus obtained, and stretching only the cellulose acylate film by 20% at 150° C. However, the method also has a problem that this manufacturing process is complicated.